This invention relates generally to improvements in a stereolithography method and apparatus for the production of three-dimensional objects, and more specifically, to improvements which increase the speed of and remove bottlenecks in the production of the three-dimensional objects, especially large or complex objects, without sacrificing accuracy.
Stereolithography is a process for building up a reproduction of an object layer by layer such that the layers are sequentially formed on top of one another until the overall reproduction is complete. The stereolithographic reproduction is commonly referred to as a stereolithographic object or part, or more simply, part. The process is described in more detail in U.S. Pat. No. 4,575,330, entitled "APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY," by Charles W. Hull, which issued Mar. 11, 1986, which is hereby fully incorporated by reference herein as though set forth in full. As described in U.S. Pat. No. 4,575,330, a stereolithographic apparatus ("SLA") is an apparatus for reproducing an object through the process of stereolithography. One embodiment of an SLA comprises synergistic stimulating means such as a UV laser beam or the like, photocurable liquid resin placed in a vat, and elevator means. The SLA forms each layer of a part by tracing the cross sectional pattern on the surface of the liquid resin with the UV laser beam at an exposure sufficient to cure the liquid resin to a predetermined thickness beyond the surface.
The elevator means supports the part as it is being built up, with the first layer adhering to and being supported by cured resin in the shape of webs or the like, known as a base or support, which base or support, in turn, directly adheres to the elevator means. Subsequently formed layers are then stacked on top of the first layer. As the part is being built up, the elevator means progressively lowers itself into the vat of liquid resin. At each step of the way, after a layer has been formed, the elevator means lowers that layer (along with all the other formed layers) into the vat of liquid resin so that fresh liquid resin that will be used to form the next layer flows over the previous layer. Typically, the elevator means is lowered into the liquid resin by more than the desired thickness of the next layer so that the liquid resin will flow over the previous layer rapidly. This results in excess resin (resin of greater thickness than the next layer thickness) coating over a substantial portion of the previous cross section. The elevator means is then raised and one or more techniques of decreasing the excess resin thickness is implemented so that a coating thickness of depth equal to the next desired layer thickness is achieved. At some point during the process, the upper surface of the previously cured cross section is positioned to be a depth below the liquid surface equal to the next desired layer thickness. This prepares the surface of the resin and position of the previous layer for exposing the next cross section and adhering it to the previous layer. The various methods of decreasing the thickness of the excess resin are disclosed in several of the following co-pending patent applications.
For further details on stereolithography, reference is made to U.S. Pat. No. 4,575,330 and the following pending U.S. and international patent applications, which are incorporated herein by reference in their entirety, including appendices attached thereto or material incorporated therein by reference, as if fully set forth:
U.S. patent application Ser. No. 339,246, filed Apr. 17, 1989, entitled "STEREOLITHOGRAPHIC CURL REDUCTION"; PA1 U.S. patent application Ser. No. 331,644 , filed Mar. 31, 1989, entitled "METHOD AND APPARATUS FOR PRODUCTION OF HIGH RESOLUTION TEE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. patent application Ser. No. 183,015, FILED Apr. 18, 1988, entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY," now U.S. Pat. No. 5,015,424; PA1 U.S. patent application Ser. No. 182,801, filed Apr. 18, 1988, entitled "METHOD AND APPARATUS FOR PRODUCTION OF TEE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY", now U.S. Pat. No. 4,999,143; PA1 U.S. patent application Ser. No. 268,429, filed Nov. 8, 1988, entitled "METHOD FOR CURING PARTIALLY POLYMERIZED PARTS"; PA1 U.S. patent application Ser. No. 268,428, filed Nov. 8, 1988, entitled METHOD FOR FINISHING PARTIALLY POLYMERIZED PARTS", now abandoned; PA1 U.S. patent application Ser. No. 268,408, filed Nov. 8, 1988, entitled "METHOD FOR DRAINING PARTIALLY POLYMERIZED PARTS", now abandoned; PA1 U.S. patent application Ser. No. 268,816, filed Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR PROFILING A BEAM"; PA1 U.S. patent application Ser. No. 268,907, filed Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR CORRECTING FOR DRIFT IN PRODUCTION OF OBJECTS BY STEREOLITHOGRAPHY"; PA1 U.S. patent application Ser. No. 268,837, FILED Nov. 8, 1988, entitled "APPARATUS AND METHOD FOR CALIBRATING AND NORMALIZING A STEREOLITHOGRAPHIC APPARATUS"; PA1 U.S. patent application Ser. No. 365,444, filed Jun. 12, 1989, entitled "INTEGRATED STEREOLITHOGRAPHY"; PA1 International patent application Ser. No. PCT/US89/04096, filed Sep. 26, 1989, entitled "RECOATING OF STEREOLITHOGRAPHIC LAYERS." PA1 U.S. patent application Ser. No. 265,039, filed Oct. 31, 1988, entitled "APPARATUS AND METHOD FOR MEASURING AND CONTROLLING THE LEVEL OF A FLUID", now abandoned; PA1 U.S. patent application Ser. No. 249,399, filed Sep. 26, 1988, entitled "METHOD AND APPARATUS FOR PRODUCTION OF THREE-DIMENSIONAL OBJECTS BY STEREOLITHOGRAPHY"; and PA1 U.S. patent application Ser. No. 427,885, filed concurrently herewith, entitled "STEREOLITHOGRAPHIC APPARATUS."
In this embodiment of the SLA, the UV laser beam is typically produced by a HeCd laser having a maximum power of 50 mW, and a wavelength of about 325 nm. Higher power lasers were not thought possible because of the problem of curing spurious liquid resin when the laser beam was being positioned (crossing over regions that should remain unpolymerized) between vectors (it is intended that curing only occur at locations corresponding to vector data). The laser is also typically kept stationary with respect to the vat, and the beam from the laser directed by means of an optical system, including a pair of rotatable dynamic mirrors, to follow an optical path to the surface of the resin, and thereafter trace out part layers on the surface of the liquid resin through the controlled rotation of the dynamic mirrors. In this embodiment, the dynamic mirrors are normally 2-mirror, 2-axis galvanometer scan heads which operate along two substantially perpendicular axes to generate a tracing of the laser beam in a horizontal plane along the X and Y axis of the resin surface which is situated a substantially fixed distance from the mirrors.
The cure depth of the resin at a particular area on the resin surface will depend logarithmically on the exposure of that area by the laser beam, which, in turn, will depend on the power and intensity distribution of the laser beam as well as the scanning velocity of the dynamic mirrors as the beam passes over that area. The more powerful the laser (the higher the intensity), or the slower the mirrors, the greater the exposure. At present, as mentioned earlier, the maximum power of the HeCd lasers used in an SLA is 50 mW, and the velocity of the dynamic mirrors can range from between about 0 to 30 in/sec., while controllably exposing a surface.
In the course of building up a cross-section of a part, it is sometimes necessary to position the laser beam by causing the laser beam to jump over certain areas of the resin surface, such as hollow areas, without polymerizing any appreciable amount of resin (enough to gel). This is accomplished in the embodiment described above by limiting the power of the laser so that at the maximum velocity of the mirrors, the exposure will not be sufficient to cure any appreciably amount of resin. This is one reason why the maximum laser power was limited to 50 mW.
This previous limitation to laser power was based on the scanning mirror's ability to jump the beam over regions that were not supposed to be cured. Another reason for a limited laser power was based on the maximum controllable speed of the scanning mirrors that could be effectively utilized to expose a given region. For example, if a very thin film of uniformly exposed and cured material is desired, this film will be created from a series of closely spaced effectively overlapping vectors. The intensity profile of the beam will determine how closely the vectors must be spaced. A combination of the beam power, the desired cure depth (amount of exposure required to get the cure depth), and the spacing of the vectors will determine the necessary scanning speed. If the laser power gets top high, the scanning speed required in this exemplified situation can easily exceed the maximum allowed scanning speed, thereby setting a limit on the maximum allowable laser power.
In this description, maximum laser power refers to the limiting factor when, in actuality, maximum intensity may be the limiting factor. Power and intensity are used interchangeably in this discussion since as small a beam spot size as possible is especially used in order to maintain high resolution in the X-Y dimensions. Therefore, for a small, relatively fixed spot size, intensity (power/unit area) and power cannot be easily decoupled.
Additionally, the HeCd laser discussed above for use in stereolithography are not adjustable. That is, they were either off or on thereby producing either no radiation or the maximum amount that they were capable of. Therefore, when using these lasers, it was not considered feasible to vary the beam power depending on exposure constraints.
A problem with limiting the laser power, however, is that it will substantially slow down the building of parts compared to the use of a more powerful laser. This problem becomes even more severe when the building of larger parts, typically greater than about 5 in..times.7 in..times.6 in., is attempted since these parts already take a fair amount of time to build because of their size. Using a less powerful laser will only make the problem worse. As mentioned earlier, however, a more powerful laser was not possible with this embodiment because of spurious resin solidification, the maximum scanning speed possible, and the fixed nature of the output power of the radiation. Another problem with a more powerful laser is that, at current mirror velocities, it may not be possible to form thin cross-sections, i.e., below about 5 mil. Faster dynamic mirrors are not a general solution to this problem since the accuracy of these mirrors typically degrades tremendously at velocities above about 30 in/sec.
For these and other reasons, part building with the SLA described above is now typically excessively time-consuming, especially for large or complex parts. A small but complex part, nominally 5".times.7".times.6", may take 10 to 48 hours to build with the SLA embodiment above, while a larger part, up to 20".times.20".times.24", may take even longer, up to 2 to 10 days.
Another problem with the SLA embodiment described above is that there are several bottlenecks to data flow in the overall system, which bottlenecks can become quite severe when a large or complex part is being built. This is because is these instances, quite a bit of data is required to represent the part, and this data, typically about 3 GB (gigabytes) for a large part, and dozens of megabytes for a small but complex part, must be managed and transported to various parts of the SLA throughout part building. The bottlenecks referred to above can slow down this process, and thereby further increase the time required for part building.
For example, one bottleneck is due to the need to control the dynamic mirrors, the loading of additional vectors to be drawn, the elevator, and other associated devices, in real-time, with the result that a control computer which is required to perform all these tasks will not be available to perform other important data management tasks such as CAD/CAM data conversion, which tasks may not have to be performed in real time.
Simply adding another computer to perform these background tasks, however, will not be effective, especially for large or complex parts. For example, a storage device will have to be added in order to buffer the data flow between the two computers, but the storage requirements for large or complex parts, typically 3 GB, will make this device prohibitively large and expensive.
Another bottleneck in the SLA embodiment described above is the method of interacting with the user. Before the control computer can begin building the part, it must interact with the user in order to obtain parameters for controlling part building, such as desired cure depths (which will differ from layer thickness depending on several factors), as well as parameters for controlling the physical movement of the elevator and associated devices. Collectively, these parameters are known simply as build parameters.
Various approaches to interacting with the user have been attempted. In one approach, a user is required to provide build parameters for the part as a whole. This can be very unwielding, especially for large or complex parts.
In another approach, the user is allowed to break up a part into ranges of layers, and specify different build parameters for each range. For a large or complex parts, however, it may be desirable to emphasize different qualities such as strength, aesthetics, speed, or accuracy, at different areas within a range. Requiring that the build parameters be the same for the entire range, and also requiring that the layers in these areas be grouped into the same ranges, prevents this. Thus, these approaches have not always proven successful, especially for large or complex parts.
A final bottleneck is that the SLA requires a fair amount of sophistication on the part of a user. For example, a user, to specify build parameters, must now be aware of the physical characteristics of, and implementation details of, the dynamic mirror, elevator, laser, etc. As a result, novice users cannot effectively run the system without substantial and time-consuming instruction, especially for large or complex parts. This can further slow down part building.
Thus, an object of the present invention is to provide means for rapidly building stereolithographic parts without sacrificing accuracy, especially large or complex parts.